Imagine that the taps switched off tomorrow, the rivers and streams ran dry, and the oceans turned into dry valleys. How would you react? And more importantly, how long would you survive?

There’s no reliable predictor of how fast dehydration would kill a person. Many survival blogs suggest that an average person can survive for somewhere from two days to a week without liquids, but that’s a rough estimate at best. A person’s health, the weather and the individual’s physical activity levels all help determine how long a person will last without water. Older people, children, individuals with chronic diseases, and people who work or exercise outside are at particular risk of dehydration, according to the Mayo Clinic.

In a very hot environment, “an adult can lose between 1 and 1.5 liters [2.1 to 3.2 pints] of sweat an hour, Randall Packer, a biologist at George Washington University in Washington, D.C., wrote for Scientific American. “A child left in a hot car or an athlete exercising hard in hot weather can dehydrate, overheat and die in a period of a few hours.” [Does Caffeine Really Dehydrate You?]

Usually, when a person is dehydrated enough to get sick, they’re also suffering from overheating, meaning that the body’s internal temperature is too high.

But this isn’t always the case, especially among certain groups of people, said Dr. Kurt Dickson, an emergency-medicine doctor at Banner Thunderbird Medical Center in Arizona. Very young children and elderly people with dementia might not remember to drink water, or be able to get themselves water without help, he said.

So how much water does a person need to lose before severe dehydration sets in? According to 2009 National Health Service guidelines in the United Kingdom, severe dehydration sets in when a person loses about 10 percent of their total weight to water loss — though that measurement is too difficult to use in practice.

But at up to 1.5 liters of water loss per hour on a hot day, that kind of dehydration can happen a lot faster than conventional wisdom suggests.

Once a person’s water levels dip below a healthy amount, characteristic symptoms set in: thirst, dry skin, fatigue, light-headedness, dizziness, confusion, dry mouth, and speedy pulse and breathing, according to the University of Rochester Medical Center. Dehydrated children cry without spilling tears. Their eyes, cheeks and tummies become sunken; they grow listless, and their skin doesn’t flatten when pinched and released.

Patients come in to the emergency room, “and they’re fatigued, tired, sometimes dizzy — more when they stand up — [and] sometimes vomiting,” Dickson told Live Science. “If [the dehydration] is really bad, they can be in shock, where they’re cold and clammy, not responsive. It can also be that they just don’t feel well, a generalized malaise.”

Dickson noted that other conditions can also cause these symptoms, so it’s not always clear that dehydration is the culprit. “You’ve got to rule other things out,” he said. “But if the guy’s a roofer and it’s July in Phoenix, you can cut a lot of things out.”

As water levels drop inside the body, the liquid gets diverted to fill vital organs with blood, causing cells throughout the body to shrink, Dr. Jeffrey Berns, then the president-elect of the National Kidney Foundation, told The Washington Post in 2014. As water leaches out of brain cells, Berns explained, the brain contracts and blood vessels within the cranium can burst.

Kidneys usually fail first among the organs and stop cleaning waste out of the shrinking blood supply, Berns said. At that point, the other organs fail in a toxic cascade. It’s a painful process, but one that’s usually easy to treat.